Protein lysine methylation contributes to modulating the response of sensitive and tolerant Arabidopsis species to cadmium stress.

The mechanisms underlying the response and adaptation of plants to excess of trace elements are not fully described. Here, we analysed the importance of protein lysine methylation for plants to cope with cadmium. We analysed the effect of cadmium on lysine-methylated proteins and protein lysine methyltransferases (KMTs) in two cadmium-sensitive species, Arabidopsis thaliana and A. lyrata, and in three populations of A. halleri with contrasting cadmium accumulation and tolerance traits. We showed that some proteins are differentially methylated at lysine residues in response to Cd and that a few genes coding KMTs are regulated by cadmium. Also, we showed that 9 out of 23 A. thaliana mutants disrupted in KMT genes have a tolerance to cadmium that is significantly different from that of wild-type seedlings. We further characterized two of these mutants, one was knocked out in the calmodulin lysine methyltransferase gene and displayed increased tolerance to cadmium, and the other was interrupted in a KMT gene of unknown function and showed a decreased capacity to cope with cadmium. Together, our results showed that lysine methylation of non-histone proteins is impacted by cadmium and that several methylation events are important for modulating the response of Arabidopsis plants to cadmium stress.

Arabidopsis thaliana plants challenged with uranium reveal new insights into iron and phosphate homeostasis.

Uranium (U) is a naturally occurring radionuclide that is toxic to plants. It is known to interfere with phosphate nutrition and to modify the expression of iron (Fe)-responsive genes. The transporters involved in the uptake of U from the environment are unknown. Here, we addressed whether IRT1, a high-affinity Fe2+ transporter, could contribute to U uptake in Arabidopsis thaliana. An irt1 null mutant was grown hydroponically in different conditions of Fe bioavailability and phosphate supply, and challenged with uranyl. Several physiological parameters (fitness, photosynthesis) were measured to evaluate the response to U treatment. We found that IRT1 is not a major route for U uptake in our experimental conditions. However, the analysis of irt1 indicated that uranyl interferes with Fe and phosphate homeostasis at different levels. In phosphate-sufficient conditions, the absence of the cation chelator EDTA in the medium has drastic consequences on the physiology of irt1, with important symptoms of Fe deficiency in chloroplasts. These effects are counterbalanced by U, probably because the radionuclide competes with Fe for complexation with phosphate and thus releases active Fe for metabolic and biogenic processes. Our study reveals that challenging plants with U is useful to decipher the complex interplay between Fe and phosphate.

Dual Targeting of the Protein Methyltransferase PrmA Contributes to Both Chloroplastic and Mitochondrial Ribosomal Protein L11 Methylation in Arabidopsis.

Methylation of ribosomal proteins has long been described in prokaryotes and eukaryotes, but our knowledge about the enzymes responsible for these modifications in plants is scarce. The bacterial protein methyltransferase PrmA catalyzes the trimethylation of ribosomal protein L11 (RPL11) at three distinct sites. The role of these modifications is still unknown. Here, we show that PrmA from Arabidopsis thaliana (AtPrmA) is dually targeted to chloroplasts and mitochondria. Mass spectrometry and enzymatic assays indicated that the enzyme methylates RPL11 in plasto- and mitoribosomes in vivo. We determined that the Arabidopsis and Escherichia coli PrmA enzymes share similar product specificity, making trimethylated residues, but, despite an evolutionary relationship, display a difference in substrate site specificity. In contrast to the bacterial enzyme that trimethylates the ε-amino group of two lysine residues and the N-terminal α-amino group, AtPrmA methylates only one lysine in the MAFCK(D/E)(F/Y)NA motif of plastidial and mitochondrial RPL11. The plant enzyme possibly methylates the N-terminus of plastidial RPL11, whereas mitochondrial RPL11 is N-α-acetylated by an unknown acetyltransferase. Lastly, we found that an Arabidopsis prma-null mutant is viable in standard environmental conditions and no molecular defect could be associated with a lack of RPL11 methylation in leaf chloroplasts or mitochondria. However, the conservation of PrmA during the evolution of photosynthetic eukaryotes together with the location of methylated residues at the binding site of translation factors to ribosomes suggests that RPL11 methylation in plant organelles could be involved, in combination with other post-translational modifications, in optimizing ribosome function.

The plasma membrane-associated cation-binding protein PCaP1 of Arabidopsis thaliana is a uranyl-binding protein.

Uranium (U) is a naturally-occurring radionuclide that is toxic to living organisms. Given that proteins are primary targets of U(VI), their identification is an essential step towards understanding the mechanisms of radionuclide toxicity, and possibly detoxification. Here, we implemented a chromatographic strategy including immobilized metal affinity chromatography to trap protein targets of uranyl in Arabidopsis thaliana. This procedure allowed the identification of 38 uranyl-binding proteins (UraBPs) from root and shoot extracts. Among them, UraBP25, previously identified as plasma membrane-associated cation-binding protein 1 (PCaP1), was further characterized as a protein interacting in vitro with U(VI) and other metals using spectroscopic and structural approaches, and in planta through analyses of the fate of U(VI) in Arabidopsis lines with altered PCaP1 gene expression. Our results showed that recombinant PCaP1 binds U(VI) in vitro with affinity in the nM range, as well as Cu(II) and Fe(III) in high proportions, and that Ca(II) competes with U(VI) for binding. U(VI) induces PCaP1 oligomerization through binding at the monomer interface, at both the N-terminal structured domain and the C-terminal flexible region. Finally, U(VI) translocation in Arabidopsis shoots was affected in pcap1 null-mutant, suggesting a role for this protein in ion trafficking in planta.

Calcium-permeable cation channels are involved in uranium uptake in Arabidopsis thaliana.

Uranium (U) is a non-essential and toxic element that is taken up by plants from the environment. The assimilation pathway of U is still unknown in plants. In this study, we provide several evidences that U is taken up by the roots of Arabidopsis thaliana through Ca2+-permeable cation channels. First, we showed that deprivation of Arabidopsis plants with calcium induces a 1.5-fold increase in the capacity of roots to accumulate U, suggesting that calcium deficiency promotes the radionuclide import pathway. Second, we showed that external calcium inhibits U accumulation in roots, suggesting a common route for the uptake of both cations. Third, we found that gadolinium, nifedipine and verapamil inhibit the absorption of U, suggesting that different types of Ca2+-permeable channels serve as a route for U uptake. Last, we showed that U bioaccumulation in Arabidopsis mutants deficient for the Ca2+-permeable channels MCA1 and ANN1 is decreased by 40%. This suggests that MCA1 and ANN1 contribute to the absorption of U in different zones and cell layers of the root. Together, our results describe for the first time the involvement of Ca2+-permeable cation channels in the cellular uptake of U.